It is a common practice to spray an insecticide on field corn during early tasseling when making a fungicide application.  

I often get no answer when I ask a grower what pest he is spraying.

Just as commonly I’m told they are trying to control corn earworm, Japanese beetles or corn borers. This application is often made without regard to pest populations, and thus even if needed, is poorly timed.

So when does it pay to spray tasseling field corn with insecticide? There is little question that treating for southwestern or European corn borers in non-Bt corn can improve yields if corn borers are present in high enough numbers. 

I have many examples of 10-40 bushel per acre yield increases from one application in the presence of moderate to high populations. This is why I so strongly promote the use of southwestern corn borer pheromone traps around fields of non-Bt corn.

Insecticides such as Belt, Prevathon, Besiege and Intrepid provide better residual control than the pyrethroid insecticides. Thus, you may not be penalized as severely for poorly timed applications (providing they are not too late). 

However, it is common to have low corn borer infestations even in non-Bt corn. Don’t spray a field just because it is non-Bt. I’ve recently written an article about interpreting trap catches and managing corn borers in non-Bt corn (link here).

So what about Bt corn? I really struggle here to find a good reason to spray Bt corn.  This should be the exception and not the rule. 

The Bt technologies take corn borers out of the equation, providing near perfect control. Japanese beetles may very occasionally do enough silk clipping to justify treatment (see related article).  

Many Bt corn technologies provide pretty good control on populations of fall armyworm or corn earworm, and let’s face it, fall armyworm is really only a common problem on late planted corn in Tennessee. 

And a one-time treatment for corn earworm with the intent of reducing kernel injury just doesn’t work. I’ve tried repeatedly and so have many others (for decades). It takes multiple applications, regardless of the insecticide you choose, to reduce corn earworm populations and kernel damage within the ear.